Ready to put out some fires

Posted by Erica on Apr 11, 2013 in Making |

Finally the time has come — the Abington Firefighting Competition is two days away. FireCheetah is tested and ready to go! The kids have their own robot this year, a LEGO Mindstorms creation named Dark Lord, and he is looking good too.

All about sensors

With navigation mostly solved early on, there was more time to look at the sensor side of things. There is more to it than I initially realized. Take candle detection, for instance. I thought my homemade UV sensor was flaky due to varying values in the same location. Turns out that was due to changes in the abundant sunlight in my testing area. I added a sensor hood to reduce the effect, but even so the ambient UV level caused by the sun is significant. I need to change my threshold for fire detection by a factor of two depending on whether I am testing in the day or at night. (This caused some serious hair pulling until I figured it out.)

The sonar, also, has its quirks. Every now and then, a wrong value creeps in. This is particularly bad for times when I use the side sensors the line up with the wall. Both values need to be accurate. In the end I was able to resolve this by making sure that two subsequent readings for each sonar are consistent and within a valid range. If the result yields an alignment change of over 45 degrees I discard it completely, and resulting turns are capped to 30 degrees. It’s worth noting here that common sensor library functions like “take an average” or “use the median” don’t solve this problem.

IR sensors are famous for variability but strangely enough I have had few problems with mine. Still, the precise values for the maze walls on Saturday may be different.

I plan to spend most of my morning practice time in the arena calibrating the sensors, especially for the candle. I won’t line up for the maze practice until later. Last year many robots navigated the course successfully and failed to find the candle, which could certainly happen if I don’t calibrate properly.

And many things depend on battery level. I plan to run with a full battery, so that is what I try to test with.

The sensor lessons confirmed the value of copious logging combined with repetitive testing. It’s a little boring — the kids told me many times, “Mom, your robot is done!” — but it’s the best way to drive out bugs. Below is a picture of the maze I used:

Robot testing maze

Robot testing maze. This took up most of my family room for a few weeks.

Audio start

Starting the robot with a 3-4 kHz sound, instead of a button, is good for a 20% time bonus. I figured this would be fairly simple, and I tried to build a circuit with an LM567 tone detector chip. Everyone says this is easy…well, after trying all the capacitors I had, and laying in a stock supply of new ones with all the right values, I never got it working. Plus, it was pretty darn ugly. Circuits with lots of components are just asking for trouble.

After two days of frustration I found a better way. With an electret microphone, a resistor, and an Arduino DTMF decoder library, the problem was solved. Honestly I hate RC circuits and analog electronics, not surprising that software worked better. This trigger has been flawless. Never a false start, and the robot responds to the tone before my ears do! I use the TrueTone app on my phone to provide the sound.

Putting out the candle

You get a 25% bonus for using something other than air to extinguish the candle. I am sticking with air, specifically a fan, because you get to try more than once if you don’t succeed the first time!

Returning to the start

Another 20% bonus is available if your robot returns to the start after putting out the fire. Also, it looks very cool. This turned out to be pretty easy. I already have a planner, so I simply needed to add a list of return path nodes from each room. When the robot enters a room, it saves the entry point coordinates. It moves around to get the fire out. Then it uses dead reckoning to find its way back to the entry point, facing the opposite direction.

This is the only time I use dead reckoning. Although I’d use it elsewhere in principle, the sensor methods proved to be more reliable. They are less sensitive to slip from carpets or variable maze dimensions.


Homemade robots are delicate things. Things can shake loose or move over time. I’ve done what I can to make things as solid as possible, but I will have to use a checklist before each run just to be sure everything is right. The left hub setscrew can come loose, it’s rare, but when that happens it’s a disaster.

The drive train can definitely get over the carpet now, unless the set screw is loose. Of course my carpets may not be the same as what is present at the competition, but from my old videos they look similar. And speaking of carpets…

A last-minute rules change

Yesterday all the contestants received an e-mail notice that carpets would now be optional in the high school and senior division, rather than required, due to “confusion in the rules”. What???

Last year, the carpets were harbingers of doom for many robots. So much so, that any robot who made it through twice was guaranteed a prize; and my design reflects that, with lots of sensor checks and — therefore — a slower speed. A carpet-free robot can use dead-reckoning and potentially be very fast. Of course, this is not the cutthroat world of Trinity. Nevertheless I suspect many teams will take this option and that means more competition.

Using the carpets does come with a significant time bonus, 30% off. That is not as great as the multi-room mode bonus (not a bonus exactly, you just avoid a 3 minute penalty). Unless I show up for practice and find the carpets far worse than my test ones, I will use the carpets. After all that is what I designed for and want to conquer, and what is life without some risk. Audio start, carpets, and return home combine to form a 0.45 factor applied to total time.

What’s a robot blog post without some video? Here is FireCheetah navigating the full mockup of the Abington/Trinity maze, putting out the candle, and returning home. One take, I swear.

Whatever happens on Saturday, preparing for the competition was a wonderful experience and I learned a great deal. I’m looking forward to seeing some of the people I spoke with last year. This time I hope to relax and enjoy and watch more of the competition.

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